1. Technical Field
The present invention relates to a longitudinal force control apparatus. Moreover, the present invention also relates to a saddled vehicle having a longitudinal force control apparatus.
2. Description of the Related Art
The mechanism by which a lateral skid of a wheel occurs while a vehicle is curving is explained by reference to a friction circle. A friction circle is a circle which indicates maximum allowance for a gripping force of a wheel. FIG. 15 and FIG. 16 show relationships between forces acting on a wheel and a friction circle.
FIG. 15 shows a relationship when a vehicle is curving while maintaining a stable attitude. As shown in FIG. 15, it is when a resultant force of a longitudinal force and a cornering force (lateral force) occurring in a lateral direction (right-left direction) during curving fits within a friction circle that a wheel rotates stably without skidding. The longitudinal force is a sum of a driving force acting in the forward direction and a braking force acting in the rearward direction.
On the other hand, FIG. 16 shows a relationship when a lateral skid occurs while a vehicle is curving. As shown in FIG. 16, if the resultant force of the longitudinal force as a sum of the driving force and the braking force and the cornering force occurring during curving goes out of the friction circle, a wheel undergoes a lateral skid. A lateral skid will disturb the attitude of the vehicle.
For four-wheeled vehicles, longitudinal force control apparatuses for controlling the longitudinal force while curving are known. A saddled vehicle, unlike a four-wheeled vehicle, will curve with its vehicle body banking, however. Therefore, a longitudinal force control apparatus for a four-wheeled vehicle cannot be straightforwardly applied to a saddled vehicle.
Accordingly, a longitudinal force control apparatus for saddled vehicles is proposed in Japanese Patent No. 4402379 (hereinafter “Patent Document 1”). The apparatus of Patent Document 1 includes a lateral acceleration sensor and a yaw rate sensor mounted on a vehicle. This apparatus evaluates an output signal from the lateral acceleration sensor to determine whether the braking force is to be decreased or not. Based on an output signal from the yaw rate sensor, it is determined on which wheel the braking force is to be decreased.
However, in the apparatus of Patent Document 1, the lateral acceleration sensor which is mounted on the vehicle will tilt along with the vehicle during curving, thus being unable to detect an accurate lateral acceleration (i.e., an acceleration in the horizontal direction, which is orthogonal to the direction of gravity). This leads to a problem in that the detected value of lateral acceleration will vary with the bank angle.
FIG. 17 is a diagram for describing a detection value of the lateral acceleration sensor in the apparatus of Patent Document 1, illustrating a case where the vehicle body 62 is banked. When the vehicle body 62 is not banked, the lateral acceleration sensor can accurately detect acceleration along the horizontal direction. However, when the vehicle body 62 is banked to a certain angle (bank angle) β, as shown in FIG. 17, the lateral acceleration sensor will detect an acceleration along a direction (an orthogonal direction to the vehicle body center axis) which is inclined by angle β with respect to the horizontal direction.
Therefore, in the apparatus of Patent Document 1, the detection value of the lateral acceleration sensor will vary with the magnitude of the bank angle β during curving. As a result, the accuracy of detection of lateral acceleration will fluctuate, such that detection of a lateral skid of the wheel 61 becomes more difficult as the bank angle β increases.